Some GRIN material is made in the form of a doughnut. The inside diameter (the diameter of the hole in the doughnut) is 34 cm and the outside diameter is 46 cm; the cross section of the ring, therefore, is 6.0 cm in diameter.
(a) If the average index is 1.6, what radial gradient is needed to keep a beam of light traveling along exactly in the center of the ring?
(b) What should be the highest, and the lowest refractive index of the material?